In all coating methods, and especially in the curtain coating method, problems are created by the air which is carried along to the coating point by the uncoated support due to friction. The coating point designates the location where the coating liquid first contacts the support. In many coating methods, and also in the curtain coating process, this location has the form of a line and is referred to as the dynamic wetting line. The area near the support where the air is in motion due to friction is called the boundary layer.
In the curtain coating process, the following problems with respect to the air boundary layer are known:
Coating means replacing the air on the support by a liquid. If this cannot be achieved, which may be the case mainly at increased coating speeds, air is entrained between the support and the liquid film, and a coherent coating is no longer possible.
Yet, even if the air is not entrained between the support and the liquid film, it strikes the back of the curtain, seen in the direction of motion of the support, with a considerable force, especially in the case of high coating speeds. This leads to disturbances, mainly in the area of the dynamic wetting line, which cause diffuse irregularities in the coated film.
In one way or another, the air which is carried against the front side of the curtain by the support must be removed, namely sideways, upwards, backwards, but not in the direction of motion of the support. Especially in the case of wide supports, this removal of the air creates problems whereby the curtain is blown up in the direction of motion of the support like a balloon. On one hand, a deformation of the curtain results, whereby the quality of the coated film may be impaired, and on the other hand, a deformation of the dynamic wetting line. This results in an irregular coating behavior of the curtain transversely to the coating direction, which may again impair the quality of the coated film, e.g. in the form of air inclusions in the marginal areas of the curtain.
Different methods and devices have been developed in order to eliminate or at least reduce the negative effects described above. In the method and the device according to the preamble of claims 1 and 3, i.e. according to U.S. Pat. No. 5,624,715, on one hand, the boundary layer is blocked by a body which is disposed in front of the curtain as seen in the direction of motion of the support and at a small distance from the coating roll, and on the other hand, the air which flows through the gap created between the body and the support is extracted by a slit nozzle while air is supplied at this location in a controlled manner.
Also, according to EP-B-0 489 978, a shield which is disposed concentrically around the coating roll is used in order to mechanically stop the boundary layer. Additionally, in this case, the air flowing through the concentric gap is extracted in the center of the body.
The main effect of both methods described above is based upon the mechanical obstruction of the air boundary layer. Although air is additionally extracted in both devices, a considerable amount of air still strikes the back side of the curtain, so that losses in quality as described above may still occur.
In both apparatus, the design of the suction device, in particular, is ineffective due to its slit configuration. According to EP-B-0 489 978, a relatively large chamber is additionally provided between the suction slit and the concentric gap where the air is strongly swirled, so that the residual air strikes the curtain with irregular forces in the direction transversely to the movement of the support and may thus cause losses in quality. Moreover, since the suction device is disposed at the center of the body, the air is aspired from the space between the body and the curtain to the concentric gap in opposition to the movement of the support. In front of the curtain and particularly near the dynamic wetting line, this suction creates an additional air flow which is irregular transversely to the movement of the support and which may therefore result in a loss in quality of the coated film.